Halogen assisted physical vapor transport method for silicon carbide growth

ABSTRACT

A physical vapor transport growth technique for silicon carbide is disclosed. The method includes the steps of introducing a silicon carbide powder and a silicon carbide seed crystal into a physical vapor transport growth system, separately introducing a heated silicon-halogen gas composition into the system in an amount that is less than the stoichiometric amount of the silicon carbide source powder so that the silicon carbide source powder remains the stoichiometric dominant source for crystal growth, and heating the source powder, the gas composition, and the seed crystal in a manner that encourages physical vapor transport of both the powder species and the introduced silicon-halogen species to the seed crystal to promote bulk growth on the seed crystal.

BACKGROUND

The present invention relates to the bulk growth of large, high-quality,silicon carbide crystals for electronic and related applications.

Silicon carbide (SiC) is a compound of significant interest as amaterial for both substrates and active layers for high voltage and highfrequency semiconductor devices as well as being of interest in themanufacture and structure of certain types of light emitting diodes.

From an electronic standpoint, silicon carbide has a number oftheoretical and practical advantages that make its use desirable inmicroelectronic devices. Silicon carbide has a wide band gap, a highcritical breakdown field (approximately two mega-volts per centimeter),and a high thermal conductivity (about five watts percentimeter-Kelvin). Silicon carbide is also physically very hard.Silicon carbide has a high electron drift velocity, excellent thermalstability, and excellent radiation resistance or “hardness.” Theseadvantages have been recognized and described thoroughly in the patentand non-patent literature.

As is the case with other semiconductor materials, silicon carbide canbe grown as “bulk” crystals or as epitaxial layers. Bulk growthgenerally (although not necessarily exclusively) refers to growthtechniques that produce larger crystals for use as substrates andrelated purposes. Bulk growth techniques, although not necessarily“fast” in an absolute sense, generally proceed at a rate sufficient tomake the techniques economically worthwhile and the resulting bulkcrystals economically obtainable. Bulk crystal growth typically refersto growth from a seed, but as used herein can also refer to the growthof layers which are sufficiently thick to share the functionalcharacteristics of bulk grown crystals.

By way of comparison, epitaxial growth is typically used to producesmaller portions—most often layers—of a semiconductor material with highpurity, high crystal quality, and specific doping parameters. Relativelyspeaking, epitaxial growth generally proceeds more slowly than bulkgrowth, but produces a higher quality crystal. Furthermore, becauseepitaxial layers can serve their purpose even when relatively thin, thelonger time required to grow them is acceptable in exchange for theirhigher quality.

Bulk growth of silicon carbide is typically carried out by one of twomethods: sublimation from a source powder or high temperature chemicalvapor deposition (HTCVD).

The HTCVD technique uses a seed crystal, but instead of a siliconcarbide source powder, silicon containing species (typically silane) andcarbon containing species (typically propane) are introduced as gases.

The HTCVD technique can produce high purity, highly uniform materialwith controlled electronic characteristics. Nevertheless, longer(larger) crystals are hard to obtain because the growth efficiency isrelatively low and parasitic reactions compete with the desired siliconcarbide growth. Additionally, silane tends to decompose in significantamounts at relatively low temperatures (in some cases below 400° C.) ascompared to those needed for bulk SiC growth (e.g. about 2000° C.).HTCVD suffers from other disadvantages including the tendency of thereaction products to deposit everywhere—i.e. throughout the depositionchamber as well as on the desired surface—which wastes material andrequires the deposition apparatus to be cleaned frequently.

Furthermore, the displacement reactions characteristic of HTCVDtypically generate hydrogen as a reaction product. In turn, hydrogenwill tend to etch silicon carbide at HTCVD temperatures.

Sublimation, also referred to as physical vapor transport (PVT), isusually carried out in the presence of a seed. In this technique, a seedcrystal of silicon carbide and a silicon carbide source powder are bothplaced into a crucible (typically formed of graphite). The crucible isthen heated in a manner that creates a temperature gradient between thesource powder and the seed, and with the powder generally being warmerthan the seed. At appropriate temperatures (i.e. at least about1900-2000° C.), silicon carbide source powder will sublime to formgaseous species (dominated by Si, Si2C and SiC2). The temperaturegradient encourages the species to migrate to the seed, which istypically maintain about 100-200° C. cooler than the source powder. Themigrating species condense on the seed crystal providing the desiredcrystal growth.

Although relatively well understood and well-established (e.g., commonlyassigned U.S. Pat. No. 4,866,005) the static use of source powder in aclosed crucible can limit the quality of the crystal eventuallyproduced.

In this regard, it will be understood by those familiar with electronicdevices and semiconductor materials that the term “quality,” is appliedin a relative sense. Sublimation produces very high-quality siliconcarbide crystals for many purposes. Nevertheless, when SiC devices areused at extremely high power—which represents one of SiC'sadvantages—even a small number of defects can degrade performancenoticeably or even lead to device breakdown. Thus, increasing thequality of silicon carbide bulk crystals always remains of interest.

As one particular problem, and because of the thermodynamic differencesbetween silicon and carbon, silicon carbide tends to sublime in anon-stoichiometric fashion. Although the mechanism is not totallyunderstood, the silicon content of the source powder tends to depletemore quickly than the carbon content. This produces a carbon-rich sourcepowder, a characteristic referred to as “powder graphitization.” Evensource material that is intentionally made or selected to besilicon-rich becomes graphitized over time.

In turn, powder graphitization causes the ratio of the vaporized siliconand carbon species to change during any given growth run. Such changescan produce undesired changes in the growing crystal. For example,higher silicon-to-carbon ratios tend to produce the 3C polytype ofsilicon carbide even when the 6H polytype is being used as the seed.

As another potential factor the composition of transported gases thatproduces the best initial nucleation on the seed crystal may bedifferent from the composition that produces the best bulk growth (andvice versa). Thus, in the conventional physical vapor transport(sublimation) systems, neither nucleation nor bulk growth may beoptimized. Instead, both may be compromised based upon the fixedstarting material.

Stated differently, in conventional physical vapor transport growthtechniques, the relevant system is loaded with a starting material andthen heated to drive the sublimation growth of the resulting crystal.The application of heat, however, is typically the only step that can bemanipulated during the growth process; i.e., the starting materials arelocked in and cannot be modified as growth proceeds.

Other problems exist. For example, where nitrogen is used as a dopant tocreate n-type material, the normal and expected process is for thenitrogen dopant atoms to replace carbon atoms in the crystal structure.Changing the ratio of silicon-to-carbon, however, causes the nitrogen tocompete with a different amount of carbon for a given position in thegrowing crystal. This, among other factors, can result in intrinsicdefects such as silicon vacancies and carbon vacancies. Furthermore, itis generally expected that the formation (or prevention) of micropipesis affected by the silicon to carbon ratio in the vapor phase.

Additionally, these growth issues are of greater concern as the diameterof the growing crystal increases. In this regard, in a commercialcontext growing larger diameter crystals is usually more efficient thangrowing smaller diameter crystals. In silicon-based technology, wafersas large as eight inches (200 millimeters) in diameter are commerciallyavailable and widely understood. In silicon carbide technology, however,three and four inch wafers (75-100 mm) still remain as a commercialupper limit.

Accordingly, interest continues to exist in improving the techniques forbulk growth of silicon carbide end in correspondingly improving theresulting bulk crystals.

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe followed detailed description taken in conjunction with theaccompanying drawings.

SUMMARY

In one aspect the invention is a physical vapor transport growthtechnique for silicon carbide. The method includes the steps ofintroducing a silicon carbide powder and a silicon carbide seed crystalinto a physical vapor transport growth system, separately introducing aheated silicon-halogen gas composition into the system in an amount thatis less than the stoichiometric amount of the silicon carbide sourcepowder so that the silicon carbide source powder remains thestoichiometric dominant source for crystal growth, and heating thesource powder, the gas composition, and the seed crystal in a mannerthat encourages physical vapor transport of both the powder species andthe introduced silicon-halogen species to the seed crystal to promotebulk growth on the seed crystal.

In another aspect, the invention includes the steps of heating a siliconcarbide source powder to sublimation temperatures in the presence of asilicon carbide seed crystal that is maintained at a cooler temperaturethan the silicon carbide source powder to encourage physical vaportransport between the source powder onto the seed crystal to cause theseed crystal to grow, and during sublimation of the source powder andsublimation growth of the seed crystal, introducing a silicon-halogengas composition in the presence of the seed crystal and the siliconcarbide source powder to moderate or eliminate the variations in thestoichiometry of the gas species that would otherwise occur in thepresence of silicon carbide source powder alone.

In another aspect, the invention is a system for bulk growth of siliconcarbide. In this aspect, the invention includes a graphite crucible, aseed crystal in the graphite crucible, a silicon carbide source powderin the graphite crucible, a source of silicon-halogen gas composition,and an inlet for introducing a silicon-halogen gas composition into thecrucible at a position at which the silicon-halogen gas composition canreact with gas species generated by the source powder rather thandirectly with the source powder itself.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of calculated equilibrium amounts of various species ina silicon, carbon, and chlorine system at physical vapor transportcrystal growth temperatures.

FIG. 2 is a comparison between the calculated equilibrium amounts of SiCformed in a reaction zone and the experimental growth rates as afunction of the amount of SiCl₄ present.

FIG. 3 is a plot of SiC growth rate as a function of SiCl₄ flow in thepresence of a fixed amount of propane.

FIG. 4 is a schematic diagram of a growth system useful in the presentinvention.

DETAILED DESCRIPTION

The invention is a physical vapor transport growth technique for siliconcarbide that preserves the advantages gained from using a siliconcarbide source powder while moderating or eliminating the issues raisedby graphitization of the source powder. The technique includes the stepof introducing a silicon carbide powder and a silicon carbide seedcrystal into a physical vapor transport growth system. The use of thesilicon carbide powder differentiates the invention from some of thehigh temperature gas source techniques referred to elsewhere herein.

The silicon-to-carbon ratio in the growth system is controlled byseparately introducing a heated silicon-halogen gas composition is intothe system in an amount that is less than the stoichiometric amount ofthe silicon carbide source powder. The silicon carbide source powderremains the stoichiometric dominant source for crystal growth.

The source powder, the gas composition and the seed crystal are heatedin a manner that encourages physical vapor transport from both thepowder source and the introduced silicon-halogen species to the seedcrystal to promote bulk growth on the seed crystal.

To date, the effects of the added halogen-containing species areempirical. Thus, it appears for example, that a lower ratio of siliconto carbon in the vapor phase encourages unwanted 15R polytype inclusionsin a growing 6H crystal. In such a case, the added silicon-halogencomposition provides additional silicon to moderate the vapor speciestowards a more favorable Si:C ratio.

Those persons of ordinary skill in this art will recognize that thegeneral principles of sublimation or physical vapor transport growth arealways applied in the context of specific and individual equipment andcircumstances. Thus, such persons will understand that the descriptionsherein are exemplary rather than limiting and such persons will be ableto carry out the techniques described herein without undueexperimentation.

Additionally, the physical vapor transport or sublimation growth ofsilicon carbide has been addressed in a number of issued U.S. patentsincluding U.S. Pat. Nos. 4,866,005; RE34,861; 6,749,685; 7,147,715;7,192,482; 5,679,153; and 6,974,720 the contents of each of which isincorporated entirely herein by reference. These are exemplary, ratherthan limiting of the information available concerning bulk growth ofsilicon carbide.

FIGS. 1, 2, and 3 provide information about the behavior of silicon,carbon, and chlorine (including ions of each and compounds of each) atthe temperatures generally used or approached for physical vaportransport growth of silicon carbide. FIG. 1 is taken from Nigam, Growthkinetics study in halide chemical vapor deposition of SiC, J. CRYSTALGROWTH 284, pages 112-22 (2005). FIG. 1 shows the calculated equilibriumamounts of dominant species in a system of silicon tetrachloride andargon as a function of temperature at a pressure of 200 torr. The inputamounts of argon, silicon tetrachloride and carbon used in thecalculations were fixed.

FIG. 1 shows that silicon tetrachloride remains stable (i.e.,undissociated) at temperatures below about 1800° C. and then dissociatesinto several of the other species shown in FIG. 1 at highertemperatures. In particular, the amount of C₂Cl₂, the most dominant ofthe carbon-containing species, is four orders of magnitude lower thanthe primary silicon containing species, which in the temperature regimeof FIG. 1 is SiCl₂.

Thus, the presence of the halogen provides stable compounds for siliconand thus provide the means for controlling the stoichiometry in thegrowth system by controlling (using) the added silicon-halogen gas.

FIGS. 2 and 3 were taken from the same source. FIG. 2 presents thecomparison between a calculated equilibrium amount of silicon carbide(the dashed line) formed in a reaction zone of a totally gas fed systemat 2050° C. and 200 ton and the experimental growth rates (blacksquares) as a function of the amount of SiCl₄ added. FIG. 2 illustratesthat at low flow rates of SiCl₄ the amount of silicon carbide formedincreases rapidly but then becomes saturated for higher flow rates(amounts) of SiCl₄ as the other source materials become depleted. Forpurposes of the invention, FIG. 2 illustrates the manner in which thegrowth rate of silicon carbide can be controlled by using small amountsof silicon tetrachloride.

FIG. 3 shows the growth rate of silicon carbide as a function of SiCl₄flow (amount) at a fixed propane flow in a fully gas fed system. Thisalso illustrates that the growth rate of silicon carbide increaseslinearly at low amounts of SiCl₄ but becomes saturated at higher amountsof SiCl₄. In turn, this illustrates the potential for controllingsilicon carbide growth rate by introducing small amounts—i.e., muchsmaller than stoichiometric—of the halogen-silicon gas.

Returning to the method of the invention, the silicon-halogencomposition is introduced into the growth system (typically a graphitecrucible) at a position that is consistent with the position of thevapor species generated by the heated silicon carbide source powder.Stated differently, the silicon-halogen composition is introduced at aposition where it can react as necessary and desired with the vaporizedspecies generated by the heated silicon carbide source powder.

In addition to silicon tetrachloride (tetrachlorosilane), alternativehalogen sources could include bromosilane (SiHBr₃), bromotrichlorosilane(SiBrCl₃), chlorosilane (SiH₃Cl), dibromosilane (SiH₂Br₂),dichlorosilane (SiH₂Cl₂), iodosilane (SiH₃I), tetrabromosilane (SiBr₄),tetraiodosilane (SiI₄), tribromosilane (SiHBr₃), tribromochlorosilane(SiBr₃Cl), trichlorosilane (SiHCl₃), triiodosilane (SiHI₃), andcombinations of these compositions.

Fluorine-containing compounds are theoretically possible but are oftendisfavored from a practical standpoint because the highelectronegativity (and thus reactivity) of fluorine makes it a candidatefor unexpected sudden reactions—including explosions—that should beavoided in laboratory and manufacturing circumstances.

The seed is introduced with a polytype that is desired for thecontinuing bulk growth, with the 6H polytype being used most frequently.Other polytypes (e.g., 4H, 15R) of silicon carbide, however, can begrown in the same manner.

The halogenated silicon composition can be partially or fullyhalogenated as may be desired or necessary in particular embodiments.Most commonly, the silicon-halogen gas is selected from the groupconsisting of SiCl₄, SiHCl₃, Si₂Cl₆ and combinations thereof.

The physical vapor transport technique is carried out below atmosphericpressure. This increases the mean free path of the various gaseousspecies and provides a greater degree of control over the variousexperimental parameters. Most typically, the total pressure in thephysical vapor transport system is maintained below about 300 torr andtypically at about 200 torr.

The physical vapor transport is typically carried out at a temperaturethat maximizes the growth rate of the bulk crystal while minimizingother factors (such as etching) that increase at higher temperatures andthat would reduce the rate of bulk growth. Generally speaking, thesource powder is heated to a temperature of at least about 1900°-2000°C. and sometimes in a range as high as 2200°-2500° C. The seed crystalis typically maintained at a temperature approximately 100°-200° C. lessthan the temperature of the source powder. This creates a thermalgradient that helps drive the relevant vaporized species from the sourcepowder to the seed crystal. As known to those familiar with sublimationgrowth, the gradient can be adjusted axially (parallel to the growthdirection) or radially (perpendicular to the growth direction).

Because the invention includes the step of separately introducing theheated silicon-halogen gas, the temperatures in the system are typicallymaintained below the thermal decomposition temperatures of such gases.In order to thermally control the growth system the silicon-halogen gasis typically preheated to temperatures approaching the temperatures ofthe seed and the source powder before introducing the silicon halogengas composition into the system. The benefit of the silicontetrachloride appears to be based on (at least theoretically) thestrength of the silicon-chlorine bond which has a dissociation energy of111 kilocalories per mole as compared to the silicon-hydrogen bond insilane which has a dissociation energy of 90 kilocalories per mole.

One of the advantageous features of the invention is the ability to usesilicon carbide source powder rather than carbon-containing gases assource materials. Thus, in exemplary embodiments the invention compriseslimiting carbon-containing source gases to gaseous species that aregenerated from the silicon carbide source powder rather than sourcegases containing carbon that are introduced separately.

If desired, a dopant-containing gas can be introduced into the system todope the bulk crystal, with nitrogen typically being introduced for thispurpose and to dope the growing crystal n-type.

The invention can also be expressed as the steps of heating a siliconcarbide source powder to sublimation temperatures in the presence of asilicon carbide seed crystal that is maintained at a cooler temperaturethan the silicon carbide source powder to encourage physical vaportransport between the source powder and the seed crystal to encouragebulk growth on the seed crystal. During sublimation of the source powderand the sublimation growth of the seed crystal a silicon-halogen gas isintroduced in the presence of the seed crystal and the silicon carbidesource powder to moderate or eliminate the variations in thestoichiometry of the gas species that would otherwise occur in thepresence of silicon carbide source powder alone.

By combining a source powder with a known or determined free Si contentwith the capacity to introduce SiCl4 or a related halogen-containinggas, the invention provides a stable and consistent (repeatable) processfor increasing the yield of high quality silicon carbide in bulkcrystals.

FIG. 4 illustrates a graphite crucible broadly designated at 10. Thecrucible is formed of a generally cylindrical wall 11, a lid 12 and afloor 13. The position of the seed crystal is illustrated at 14, and thegrowing bulk crystal 15 extends downwardly from the seed crystal 14.Silicon carbide powder is indicated by the shaded portion 16 at thebottom of the crucible 10. The crucible 10 can be heated in anyconventional manner including the use of an induction coil symbolized bythe circles 17 that inductively heats the graphite crucible 10.

In this embodiment the halogen containing gas is introduced through atube broadly designated at 20. It will be understood that the tube isillustrated schematically and that individual positions and structuresfor the tube 20 can be varied within the scope of the present invention.The tube 20 has an upper portion 21 extending above the source powder 16and a lower portion 22 that extends downwardly from the floor 13 of thecrucible 10. The desired halogen-containing gas is introduced into thecrucible 10 through the bottom portion 22 of the tube 20. The upperportion 21 extends upwardly into the crucible 10 so that heated gasesexiting the tube 20 are in position to react with vaporized speciesgenerated from the source powder 16.

In the drawings and specification there has been set forth a preferredembodiment of the invention, and although specific terms have beenemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being defined inthe claims.

1-37. (canceled)
 38. A system for bulk growth of silicon carbide, thesystem comprising: a crucible configured to receive a seed crystal and asilicon carbide source; and an inlet for introducing a silicon-halogengas composition into the crucible at a position in which thesilicon-halogen gas composition can react with gas species generated bythe silicon carbide source rather than directly with the silicon carbidesource itself.
 39. A system according to claim 1, wherein said inlet isproportionally large enough to direct a significant amount of the gasspecies to the growth zone while small enough to prevent extensive vaporloss from the silicon carbide source.
 40. A system according to claim 1,further comprising a powder silicon carbide source, wherein the inletextends into said crucible between the seed crystal and the powdersilicon carbide source to facilitate reaction of the silicon halogen gascomposition with vaporized species generated from the powder siliconcarbide source.
 41. A system according to claim 4, further configured tolimit carbon-containing source gases to the gaseous species generatedfrom the powder silicon carbide source.
 42. A system according to claim4, wherein a portion of the inlet extends into the crucible through thepowder silicon carbide source.
 43. A system according to claim 4,further configured for using a non-stoichiometric powder silicon carbidesource and/or a powder silicon-rich source.
 44. A system according toclaim 1, wherein the crucible is graphite.
 45. A system according toclaim 1, further configured to receive 100 mm seed crystal.
 46. A systemaccording to claim 1, further comprising a source of silicon-halogen gascomposition operably couplable to the inlet.
 47. A system for bulkgrowth of silicon carbide, the system comprising: a crucible configuredto receive a seed crystal and a silicon carbide source separated fromthe seed crystal; and an inlet extending into the crucible configured tointroduce a silicon-halogen gas composition into said crucible; thesystem configured to maintain the silicon-halogen gas composition in anamount that is less than the stoichiometric amount of silicon carbidesource during bulk crystal growth.
 48. A system according to claim 10,wherein said inlet is proportionally large enough to direct asignificant amount of the species to the growth zone while small enoughto prevent extensive vapor loss from the silicon carbide source.
 49. Asystem according to claim 10, further comprising a powder siliconcarbide source, wherein the inlet extends into said crucible between theseed crystal and the powder silicon carbide source to facilitatereaction of the silicon halogen gas composition with vaporized speciesgenerated from the powder silicon carbide source.
 50. A system accordingto claim 12, wherein a portion of the inlet extends into the cruciblethrough the powder silicon carbide source.
 51. A system according toclaim 12, further configured to limit carbon-containing source gases tothe gaseous species generated from the powder silicon carbide source.52. A system according to claim 10, further configured for using anon-stoichiometric powder silicon carbide source and/or a powdersilicon-rich source.
 53. A system according to claim 10, wherein thecrucible is comprised of graphite.
 54. A system according to claim 10,further configured to receive 100 mm seed crystal.
 55. A systemaccording to claim 10, further comprising a source of silicon-halogengas composition operably couplable to the inlet.
 56. A system for bulkgrowth of silicon carbide, the system comprising: (i) a crucible; thecrucible configured to: receive a powder graphite source capable ofgenerating a gas species; receive a seed crystal separated from thepowder silicon carbide source; and (ii) an inlet operably couplable to asource of silicon-halogen gas composition, the inlet having a portionextending into the crucible between the seed crystal and the powdersilicon carbide source; the system configured to facilitate reaction ofthe silicon halogen gas composition with vaporized species generatedfrom the powder silicon carbide source.
 57. A system according to claim19, further configured to limit carbon-containing source gases to thegaseous species generated from the powder silicon carbide source.
 58. Asystem according to claim 19, wherein a portion of the inlet extendsinto the crucible through the powder silicon carbide source.
 59. Asystem according to claim 19, wherein the seed crystal is 100 mm.